Not Applicable
Not Applicable
1. Technical Field
This invention relates in general to telecommunications and, more particularly, to a digital subscriber line (DSL) communications system.
2. Description of the Related Art
Data communications systems are playing an increasingly important role in society. The Internet, in particular, plays a significant role in business, education and leisure for many people, and this role is expected to expand dramatically over the next few years.
The vast majority of connections to the Internet use telephone lines and analog modems to communicate information. For many years, analog modems have been increasing in speedxe2x80x94over the last fifteen years, analog modem speeds have increased from 300 bps (bits per second) to 56 kbps (kilobits per second). 56 K modems receive data at 56 kbps (actually at 53 kbps due to telephone company specifications), but transmit data at 28.8 or 33.6 kbps.
Nevertheless, the speed increases in analog modems have been insufficient to meet the demands of users. While modems originally were used mainly to transmit text and small binary files, they are now used to transfer graphics, sound, animation and video. For telecommuting purposes, users often need to transfer large data files, as well.
Accordingly, users have been demanding greater modem speeds. An exciting prospect in providing greater bandwidth is the advent of DSL (digital subscriber line) modems. A DSL modem is one which can use existing telephone lines between the user and the telephone company""s CO (central office) to transmit data at high frequencies rather than the low frequencies used by analog modems. There are a number of DSL specifications, including ADSL (Asymmetric Digital Subscriber Line), ADSL-Lite, R-ADSL (Rate Adaptive Digital Subscriber Line), HDSL (High Speed Digital Subscriber Line), SDSL (Symmetrical Digital Subscriber Line), and VDSL (Very High Speed Digital Subscriber Line), which are referred to collectively as DSL technologies.
An ADSL modem at customer premises has the capability to receive data at 1.5-8 Mbps (megabits per second) and to transmit data at 1.544 Mbps (assuming a local loop of 12,000 feet or less for the fastest speeds). This is a significant improvement over analog modems and over other technologies such as ISDN (Integrated Services Digital Network). Unlike cable modems, which can reach similar or higher speeds, the bandwidth is dedicated to each user (cable modems share the bandwidth of the cable, the available bandwidth depends upon the number of users of the cable at a given time).
Another advantage of DSL modems is that they can be used simultaneously with a voice connection. Analog telecommunications devices, such as phones, faxes and analog modems, use the frequency spectrum between 0 and 3.4 kHz (the voice band). Many DSL technologies use a frequency spectrum above the voice band for data communication. Splitters use a low pass filter to direct the voice band frequencies to the analog telecommunication devices and use a high pass filter to direct the data band frequencies to the DSL device.
However impressive the DSL speeds are in comparison with analog modems, the near future will demand even greater speeds, as more workers telecommute and more multimedia content is available on the Internet or other global network. Therefore, a need has arisen for DSL modems which provide higher speeds, while maintaining compatibility with analog telecommunications devices.
In the present invention, a DSL telecommunications system includes a telecommunications line, a first DSL modem coupled to the line operable to communicate data over either a first or second frequency spectrum, and a second DSL modem coupled to the telecommunications line. The second DSL modem comprises detection circuitry to detect an on-hook or off-hook state of the telecommunications line and communications circuitry to communicate with the first DSL modem over the first frequency spectrum if the detection circuitry detects a on-hook state and to communicate with the first DSL modem over a second frequency spectrum if the detection circuitry detects an off-hook state.